Ion mobility spectrometry--IMS--instruments operate on the basis of measuring the time taken by ionized molecules to move through a drift region to a collector electrode, while under the influence of an electric field. Typically there is provided a counter-flowing drift gas. The length of time required for an ion to drift to the collector is a function of the size and mass of the ion, the length of the drift region, the strength of the electric field, the temperature and pressure in the drift region, and the composition of the drift gas.
In one existing design of IMS instruments, the system is operated in two modes: READY and ANALYSIS. In READY mode, the only flow in the IMS detector is a flow of drift gas which sweeps the entire length of the unit. In READY mode, when the IMS may be open to atmospheric pressure, certain calibration measurements may be taken.
In ANALYSIS mode, a sample carrier gas also flows into the IMS detector, and both the drift and sample flows exit the detector, optionally aided by suction (hereafter called exhaust flow) from an exhaust port. The sample carrier gas flow (hereafter called sample flow) transports sample molecules into the IMS where they are ionized. During ANALYSIS mode, the IMS detector is sealed from the atmosphere. Thus, unless the sum of the drift flow and sample flow exactly equals the exhaust flow during the transition from READY to ANALYSIS mode and during ANALYSIS mode, the pressure in the IMS detector may increase or decrease. This, in turn, will disrupt the accuracy of the measurements being made during the ANALYSIS mode since the calibration results established during READY mode operation no longer precisely apply. The difference between the exhaust flow, and the sum of the drift flow and sample flow is called the flow balance. In the past, system control efforts have been directed to holding the flow balance at zero.
Calibration of such a system for purposes of identifying sample molecules by their transit times is established during READY mode by the inclusion of calibration molecules of known mass (hereafter called calibrants) in the drift flow. In the READY mode, the calibrant(s) are ionized after they have traversed the drift region, and travel back through the drift region to a collector electrode while under the influence of an electric field. The time taken by calibrant ions to traverse the drift region and reach the collector is called the calibrant drift time, and is used to calibrate the system.
The calibration is established in READY mode, while the system is open to the atmosphere. As a result, the calibration is only valid in ANALYSIS mode if the IMS maintains a near-constant (i.e. atmospheric) pressure during the ANALYSIS period. If the sum of the drift flow and sample flow do not equal the exhaust flow, the pressure in the IMS may change, because the IMS is a sealed system when in ANALYSIS mode, reducing the validity of the calibration, and the accuracy of the system.
In the past, separate mass flow controllers were used to control the drift flow, sample flow, and exhaust flow in order to maintain the stability of the pressure during ANALYSIS mode. Unfortunately, mass flow controller calibration is prone to drift over time. If one or more of the mass flow controllers drifts over time, the flow balance may change, resulting in an increase or decrease of pressure in the IMS during the ANALYSIS period. In addition, pressure changes during an analysis may result from an initial pressure increase, caused by rapid heating and expansion of trapped gases, when changing from the open READY mode to the closed ANALYSIS mode. These pressure changes and instabilities will render the calibration less valid and reduce system accuracy.
An objective of this invention is, therefore, to maintain pressure in the IMS nearly constant during the transition from READY to ANALYSIS mode and during the ANALYSIS period, in order to preserve the validity of the calibration.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.